[murphyv310]

You wont brick your car with either T4 or VT4, there are built in safeguards. I don't know enough about other systems to comment.
Pete is spot on with his advice, T4 is the system that Rover used on its vehicles. VT4 is basically the same, it uses the exact same software and talks to the various ECU's in exactly the same way with the exception that you don't need all the cables and interface that came with the original T4. The difference with VT4 is purely how the Laptop/PC connects to the OBD port in your car.
Any owner who is committed and serious about his 75 or ZT will find this the best route. I obviously cannot comment about the other two systems mentioned but I would expect them to be very well developed and robust.
Just my thoughts though.